IndoArchipel-2

Background

The western tropical Pacific represents an area with strong interannual and decadal sea level variations (figure 1). While the changes in the open ocean are basically understood in terms of wind-driven redistributions of upper-ocean waters associated with major climate modes (ENSO, PDO), much less is known about the modification of these signals in the Indonesian Archipelago and the marginal seas and shelf areas of southeast Asia, i.e., about the relative role of remote and local forcing mechanisms. Projections of future trends in these areas are hampered since present climate models do not resolve the complex flow structures of the boundary currents and flows through the narrow passages, and are thus lacking potentially important processes, e.g., for connecting the changes in the western Pacific with the Indian Ocean, and for a realistic propagation of deep ocean changes onto the shelf areas and coastal waters.

Objectives

The goal of this project is to contribute to the integrated analysis of sea level change in the coastal areas and island states of South-East Asia by devising a programme of experimentation with high-resolution ocean models aiming at

• a regionally refined characterization of contemporary sea level variability over the continental shelves and marginal seas of this area;

• assessment of possible shifts in forcing mechanism of the ITF transport and related heat transport and sea level signals. Historic changes of the ITF transport can be related to changes of the wind field of the tropical Indo-Pacifc domain. In contrast to these findings recent studies based on CMIP5 projections predict a weakening of the ITF transport during the 21st century that is not related to the wind forcing of the tropical Pacific. Instead changes in the deep ocean circulation and density structure both in the Pacific Ocean and in the    Pacific domain of the Southern Ocean seem to alter the characteristics of the deep and bottom waters that is exported in the western tropical Pacific with effects on sea level tendencies and the pressure forcing between the Pacific and Indian Ocean. The exact mechanism are still unknown.

• providing regionally refined projections of the range of future regional sea level changes and their uncertainties that are to be expected from the projected trends in the atmospheric conditions simulated in 21st-  century climate scenarios (CMIP6).

Methods

The research program utilizes a hierarchy of high-resolution global ocean circulation models. Building on a global (‘eddy-permitting’) model configuration with ¼°-grid resolution (ORCA025), a novel configuration with regionally enhanced resolution of 1/20° has been developed during the first phase of the project. The high-resolution domain spans the area of interest, i.e., the South-East Asian shelf areas and Indonesian Archipelago, but extends from the western tropical Pacific to the eastern Indian Ocean, and thus includes dynamical regimes deemed important for the transformation of the basin-scale signals, such as the complex bathymetric features and boundary currents (figure 2).